Select the most appropriate adjective to fill in the blank. My sister is…

Adjectives change form to show the degree of comparison. The positive degree (for example, industrious) simply describes a quality with no comparison. The comparative degree (for example, more industrious) compares exactly two people or things, usually linked with “than”. The superlative degree (for example, most industrious) is used when one person or thing is compared to the rest of a group containing three or more members in total, and when that group is explicitly named (as in “of all”, “in the class”), the superlative form is preceded by the definite article “the”.

Here, the blank compares one sister against the entire group “of all of us” — three or more people in total — so the superlative degree is required, not the comparative. Because “industrious” is a polysyllabic adjective, its superlative is formed with “most” rather than an “-est” ending, giving “most industrious”. Since the comparison group is explicitly named by “of all of us”, the definite article “the” must precede the superlative form, giving “the most industrious”.

• “more industrious” is the comparative degree, the form used only when exactly two people or things are being compared (typically linked with “than”), not a group of three or more.

• “industrious” is the positive degree — a plain description of the quality with no comparison at all.

• “most industrious” (without “the”) uses the correct superlative form of the adjective but omits the definite article that a named comparison group such as “of all of us” requires.

So “the most industrious” — the superlative degree together with the required definite article — correctly completes the sentence.